3. 105
3. DME as a navigational aid
Distance plays a vital role for navigating from one point to other. In aviation, for locating
the position of an aircraft polar coordinates (Rho, Theta) system is used, where VOR
provides the bearing and DME the distance. Distance Measuring Equipment, or DME, is
a standard navigational aid used by all members of the International Civil Aviation
Organization (ICAO) for civilian aircraft operation. For military use, a similar system has
been developed, which is called Tactical Air Navigation (TACAN). Both operate in the
same principal.
Distance measuring is achieved by interactive communication between the aircraft and
the ground DME station. For this an aircraft initiates the process by sending a train of
paired RF modulated pulses at a rate of 135 pulse pairs per second (pps). Once the
aircraft starts getting the replies from the ground DME station the rate is reduced to
27pps. This is called "interrogation" and the aircraft is called "interrogator". After
receiving the signal the DME ground station checks the width and spacing of the
incoming signal to ascertain that they are within the specified limits. If yes, then it is
further delayed to make it exactly 50 us from the time of arrival of the first signal and
then responds back to aircraft with a similar pair of signal. This is called "reply" and the
ground station is referred to as "Transponder".
Hence DME station provides pilots with a continuous digital display of distance from the
aircraft to the facility. Operating on line-of-sight principal, DME furnishes distance
information with a very high degree of accuracy. Reliable signals may be received at
distances up to 200 NM at line-of-sight altitude with an accuracy of better than 0.5 NM
or 1.25% of the range, whichever is greater. However, the system is considered to be
capable of providing distance information accurate to within 370m (0.2NM) or 0.25%
of slant distance, whichever is greater, for at least 95% of time. Principal factors for
maximum range are aircraft height, transmitter power and receiver sensitivity, both in
ground and in the air. Distance information received from DME equipment is Slant
Range distance and not actual horizontal distance.
See fig. 1.
Slant distance
4. 106
DME station
Actual distance
(Fig. 3.1)
Most of the modern commercial jet-aircraft fly below 40,000 ft. (6.59 NM). Therefore,
when the aircraft is at a longer distance from the DME station, the slant distance and the
actual distance are very close to each other.
3.2. Distance measurement
The distance measured by the aircraft is not the horizontal distance but is the "slant"
distance. Since flying height of the aircraft compared to the distance to be measured is
relatively very small, there is virtually a very little difference to be counted for. Hence
with very negligible margin it could be considered as equal to the horizontal distance. See
the following illustration in Fig.2. When the aircraft is 100 NM away from the station and
flying at a height of 6NM (36,500 Ft) then the slant distance would be 100.18 NM which
is very close to 100NM. Accuracy is higher when the aircraft is far from the station.
However, accuracy is lower when closer to the station. Over the DME station, which is
the cone of silence, the accuracy is extremely low and cannot be used. However this
coverage is small and quickly overflown by the aircraft.
Not
accurate overhead
D = √ (62 + 1002) = 100.18 NM
6 NM
100Km
(Slant distance. Fig.3.2)
3.3 Applications
Since, DME provides distance information; it can be used in several ways in aviation.
3.3.1 DME co-located with a VOR (VOR/DME) - Rho-theta system
5. 107
It is the most popular use of DME where a DME is installed together with a VOR. Since
VOR provides azimuth and DME distance, they both form together a Rho-theta (,)
system. Thus, an aircraft can find his polar coordinate of any location around the
VOR/DME station, which acts as the center of the sphere.
This enables the pilots as well as the ground air-traffic controllers to determine the exact
position of the aircraft with respect to the station.
N
(Formation of polar coordinate by VOR/DME. Fig.3.3)
A VOR/DME station can be located at the
# Vicinity of the runway
# On the centerline of the runway
# Or, on the airway routes.
When it is installed around and on the centerline of the runway, an aircraft can use it for
homing and departure as well as to align itself to the runway, and make straight-in
approach. Such an approach, however, is not very accurate as with an ILS and is called
non-precision approach. An ILS approach is fully reliable hence it is called precision
approach. In the places where an ILS is not available, non-precision approach is very
helpful. Kathmandu airport uses DVOR/DME non-precision approach for all landings
and takeoffs day and night. When a VOR/DME station is located away from the airport,
it is mostly used for en-route aid, which provides position fix and route guidance. The
illustration in Fig. 2.3-3 shows how an aircraft can make turn at points A and B using
VOR/DME systems.
N
VOR/DME 3
N
A VOR/DME 2 (200 0NM To #3)
N (100 50NM From #2) B
VOR/DME
6. 108
(30 80NM From #1)
VOR/DME 1
(Departure, position fixing & homing using VOR/DME, Fig.3.4)
In most of the VOR/DME installations both the equipment are placed inside the same
shelter and the DME antenna is located on the same vertical axis of the VOR antenna.
It is called coaxial collocation. In DVOR/DME installations, however, due to space
restrictions the DME antenna may be installed as far as 80m from the VOR antenna
system. In some other situations, the antenna separation may be much higher, but in any
case it should not exceed 600m (2000 ft.). When antennas are separate, it is called offset
collocation.
In VOR/DME installations, DME frequency is paired with VOR as per allocations made
in ICAO Annex-10. Therefore, as soon as a pilot tunes to the specific VOR frequency, if
the DME is collocated, it is received automatically. To identify collocation, both VOR
and DME share the same station identification code. The identification code is repeated
seven times per minute, with three times for VOR and once for DME, and so on. In such
installations, the range of DME should be the same as VOR. The radiation pattern of both
equipment is Omnidirectional. If VOR is not available, DME is sometimes co-located
with a NDB. It serves the same purpose but with less accuracy with respect to azimuth
guidance. In NDB/DME installations the DME frequency is not paired. Therefore, both
have to be selected independently. In Pokhara NDB/DME collocation has been provided.
3.3.2 DME with ILS (ILS/DME)
At some geographical locations, where installation of associated Marker Beacons of the
Instrument Landing System (ILS) is not possible, a DME can be installed to provide the
distance information. When DME is used as an alternate to Markers, the DME is located
on the airport and adjusted in such a way that the zero range indication will be a point
near the end of the runway. Also, to reduce the angular error the DME antenna should not
be more than 20 from the centerline of the runway. In most of the cases, the DME is
normally located inside the Glide Slope (GS) shelter of ILS. The Glide Slope station is
normally 250 to300 meters from the end of the runway, and is only offset from the
centerline by 120-150meters. Therefore, it meets the above requirements. See fig. 2.3-4.
In ILS/DME installations, DME frequency is paired with Localizer frequency and they
both share the same identification tone, like with VOR. While it is not specifically
required that DME be frequency paired with the Localizer, in most of the cases when it is
used as an alternate to Outer Marker, frequency pairing is preferred to simplify pilot
operation.
GS/DME Outer Marker
position
(3.5 - 6 NM)
7. 109
LLZ
Zero distance
(Installation of DME with ILS. Fig. 3.5))
Where only Localizer service is provided, it can be collocated with the Localizer. DME is
also installed with the Microwave Landing System (MLS), which is an alternate to ILS
with better accuracy and ideal for difficult sites. DME collocated with an ILS or MLS
system should have directional radiation pattern with distance accuracy better than 0.2
NM.
3.3.3 DME alone
DME is also installed as an independent station. In such installations the radiation pattern
is normally omnidirectional.
3.4 Principal of operation
DME ground system, which has transmitter/receiver, called transponder, works in
conjunction with airborne transmitter/receiver, called interrogator. The principal is that
the interrogator transmits continuously a series of interrogation pulse pairs to the
transponder, which are received by the transponder receiver. After checking the
correctness of the incoming pulses the transponder holds for specified delay period and
transmits back the reply pulses. The time difference between interrogations and reply
pulses are measured in the interrogator receiver which is computed into distance
information to display directly in nautical miles. In navigation the distance is always
measured in nautical mile. 1' (minute) latitude or longitude represents 1NM. (1 NM =
1.86 KM).
8. 110
Readout in NM
Airborne
Interrogator
Ground
Transponder
(Distance Measuring System. Fig.3.6)
DME operates in UHF frequency band from 960 MHz to 1215 MHz. The band is divided
in to 126 1-MHz channels for interrogations, and another 126 1-MHz channels for
replies. There is always a difference of 63 MHz between interrogation and reply pulses.
When DME transponder is intended to operate with an ILS, VOR or Microwave Landing
system (MLS), its frequencies are paired with associated navigation system. The details
of these channel pairing is indicated in ICAO Annex-10. Thus, a pilot only tunes to ILS;
VOR or MLS frequency channels and receives automatically the distance information
when a DME is collocated with any of them. To identify a particular station, DME
transmits identification codes at a fixed repetition rate, which varies in accordance with
installations. If alone then it is at the rate of 6 words per minute.
For obtaining the distance information, it is just required to tune the VOR frequency
which will then automatically tune the DME Frequency as they are paired with each
other. The aircraft interrogator starts transmitting a series of double pulses at a Pulse
Repetition Frequency (PRF) of 135 Pulse Pairs per Second (PPS). It is called the Search
Mode. For modern interrogator equipment the search time is just 1-2 seconds. As soon as
the interrogator receives the reply signals from the ground station PRF decreases from
135 PPS to just 27 PPS and starts displaying the distance information. It is called Track
Mode or Lock Mode.
R
x
Tx
RxTx
Delay
9. 111
A modern DME station is capable of proving up to 2700 PPS. Therefore, a maximum of
100 aircraft may receive distance information simultaneously from one transponder.
However, 95 aircraft will be in lock mode and 5 in track mode. When there is no
interrogation from the aircraft, the transponder receiver generates the interrogation
signals internally at the rate of 2700 PPS and receives the reply in order to keep on
activating the transponder continuously, and to monitor the performance of the station.
As the aircraft interrogations are increased the internal interrogations are automatically
decreased at the same rate to keep overall PRF to 2700.
For interrogation as well as for reply DME uses a pair of pulses, called Gaussian Pulses,
which are 12 0.25 s apart and 3.5 0.5 s wide. The frequencies of interrogation and
reply, however, differ by 63 MHz from each other.
After receiving a pair of interrogation pulses the DME receiver checks the width and
spacing of the pulses, holds it for a total of 50 1s and then triggers back a reply.
Therefore, from the start time of the reception of the pulses the transponder receiver
would not accept any new incoming signals for 50 1s. This is called “Receiver Dead
Time”..
1
0.5
3.5 0.5s width
12 0.25s spacing
(Pulse width and spacing of interrogation and reply for DME. Fig 3.7)
The DME receiver dead time of 50 1s is necessary to make all the DME equipment
similar in performance as the actual circuit delay could vary from 20 to 30 s from
equipment to equipment that may lead to unacceptable errors. Furthermore it has
significant importance in echo suppression. This would be dealt with in detail in the
following paragraphs.
Interrogation Reply
10. 112
50 1s
(Receiver delay or dead time. Fig.3.8)
3.5 Gaussian Pulse
The DME system uses Gaussian Pulses instead of rectangular pulses, as normally is in
the case of primary Radar system. The reason for this is that the DME channels are very
closely spaced, i.e. 1-MHz apart.
If rectangular pulses were used then the frequency spectrum would follow a SinX/X form
and the energy would spread outside the 1-MHz channel bandwidth. This would cause
the energy to pass into adjacent channels, which may give rise to unnecessary
interference in co-channels.
To decrease the spectrum width it is necessary to reduce harmonics in the pulse. That's
why the Gaussian pulse has been chosen. Mathematically it can be proved that a
Gaussian pulse has relatively smaller frequency spectrum. Hence, most of the energy can
be maintained within the 1-MHz channel and interference with co-channel stations is
reduced. The Gaussian pulse can be represented by the formula:
f (x) = Ae(-t/σ)2
Where “A” is the amplitude and “σ” is the pulse half duration at 1/e point.
But this is at the expense of accuracy in distance indication, because if the detection level
will vary (which normally would occur due to shape of the pulse), it will result in
variation of time. In DME, the measurement of time is done at half amplitude of the
Gaussian pulse. Therefore, any distortion in shape may cause distance error. A variation
of just 1 s may cause an error of approximately 150 meters.
ICAO Annex-10 specifies the shape of the Gaussian pulse. The Gaussian pulse has been
illustrated in Fig.9. To reduce the harmonics and the distance error, the pulse should be
obtained in the equipment as accurately as possible.
11. 113
Amplitude
100%
90%
50%
10%
Time
Pulse rise time Pulse decay time
Pulse duration
(Gaussian pulse : Fig.-3.9)
Pulse rise time - The time as measured between 10 and 90 per cent amplitude
points on the leading edge of the pulse envelope. (2.5 - 3 s )
Pulse decay time-The time as measured between 90 and 10 per cent amplitude
points on the trailing edge of the pulse envelope. (2.5 - 3 s )
Pulse duration - The time interval between the 50% amplitude point on leading
and trailing edges of the pulse envelope. (3.5 0.5s)
12. 114
3.6 DME Transponder Operation
A simplified bloc diagram for a general DME transponder is shown in Fig. 10. The
transponder antenna, which is normally a stacked array of conical dipoles, receives
interrogation pulses. Polarization of antenna is vertical and it radiates omni-directionally
in the horizontal plain with 9dB gain at 3 degree over the horizon. The antenna works in
L-band.
13. 115
(Simplified Bloc Diagram of Transponder. Fig. 3.10)
The coupler isolates the Receiver and Transmitter signals and hence the Interrogating
pulses are passed to the Mixer, which gives 63 MHz (difference between interrogating
and reply frequencies).
The signal is amplified in the IF unit and also passes through a Ferris Discriminator
which is a very high selective Band Pass filter. Normally in the IF unit the signal is also
mixed down to a 2nd IF frequency around 11 MHz. the signal is then detected and passed
to a Decoder which checks that the pulse spacing of the so called video pulses (LF
pulses) are within 12 + 1 us. If so, the Decoder triggers a short spike pulse with
reference to the 2nd pulse in the pair.
Normally the total system delay in a modern transponder circuitry is approximately 20us
(including the 12us delay in decoder) hence the Main delay circuit must delay the pulse
spike for further 30us to obtain the over all delay of 50us. The Main delay circuit is
mainly a simple Monostable multivibrator. The Coder (or Encoder) will for each spike
input give out a double Gaussian pulse pair with the correct pulse spacing and pulse
characteristics given by ICAO. (The Coder consists mainly of multivibrators and a
Gaussian filter). This video signal modulates the Transmitter, which produces RF pulse
Antenna
Coupler
Mixer
and IF
ampl.
Decode
r
Monitor
s 1&2
Station
Identification
Main
delay Unit
Encode
r
Transmitt
er
Receive
r
Dead
time
Reply pulses fo + 63
MHZ
Interrogating
pulses (fo)
50 us
14. 116
pairs with correct frequency. The Transmitters are either Low power (100 Wp
transistorized PA) or Medium power (1kWp PA including valves) or High power (5kWp
with klystrons). Here Wp denotes “pulse power” that is much lower than the average
power of the transmitter. Pulse power is the power of the transmitter for a very short
period while transmitting the particular pulse. The frequency is always 63 MHZ above or
below the correct interrogating frequencies.
The Transponder also transmits identifications signal at every 30 seconds when co-
located with VOR or ILS-LLZ. The identification signal has the frequency 1350 pulse per
second and do have 2 or 3 letters in the Morse code which indicates the signature of the
ground beacon.
Even if no aircraft is present to interrogate the transponder the duty cycle must be kept
constant 2700 pulse pairs and this is carried out by the Monitor, which gives noise or
squitter pulses inversely proportional to interrogating pulses. Therefore, with no
interrogations all 2700 pulse pairs will come from the Monitor. On the other hand, with
100 aircrafts interrogating, there will be no pulses coming out from the Monitor, because
all 2700 pulse pairs will be produced by the aircraft.
3.7 First come first served
The pulses interrogated from the aircraft are replied one by one by the DME station. In
fact a DME station is unaware of the origin of the pulses. So long as it receives a valid
pulse (with 3.5 ±0.5 us wide and 12 ±1 us spaced) it would reply. It may also reply to the
echo (from reflections) pulses so long as they measure correctly. How does an aircraft
recognize its anticipated response? This is evident from the following example. The
interrogation rate (135 pps or 27pps) is very slow compared to timeframe allocated in a
second. The aircraft interrogates only at the rate of 135 or 27 pulse pairs in a second.
Therefore looking at the time elapsed there is huge interval between one pulse pair to
next one. See Fig.11. Suppose an aircraft at certain distance is interrogating at 27pps to
the DME station and there is x us between the pulse pairs. Then from the illustration
below:
1-st pair 2-nd pair 3-rd pair 27-th pair
x
106 us
(Spacing between interrogation pulses Fig.3.11)
27(12+x) = 106 us
Hence x = 37,000 us
15. 117
Therefore, between the interrogation pulses there is a silence period of approximately
37000 us. This time would be enough, for example, to answer 50 aircraft 50Km away
from the station before the second pair would be initiated from that aircraft. Hence, while
a particular aircraft is waiting to send another pulse pair after receiving the response,
several other aircraft would get the chance to interrogate and receive the response. Also,
an aircraft would lock to a DME station only when it would receive a series of similar
response. That is not possible to get from an interrogation by another aircraft as pulse
arrival times would not match with each other. Sometimes the echo pulses closer to
station may cause problem. But it is effectively eliminated by other techniques. This will
be dealt with later.
3.8 DME errors and echo suppression
3.8.1 DME errors
DME works in UHF band, therefore, strict line-of-sight principal applies to it. DME
mainly suffers from multi-path error. Since DME antenna in the aircraft is not
directional, the interrogation pulses from the aircraft may also be reflected from the
surrounding terrain; buildings etc. and arrive later as the echo pulses. See Fig. 12. The
echo pulses, if they are strong and within the specified limits (i.e. correct width and
spacing), they may also be accepted by the transponder as the true signals. Consequently,
false replies may be triggered back. These replies originating from echo pulses could be
accepted in some aircraft receivers and may cause false indications.
Tower
t2
t
rock
t1
t3
DME station House
16. 118
(Formation of echo pulses. Fig. 3.12)
3.8.2 Echo suppression by DME dead time
To eliminate echo to some extent, DME dead time is very useful. The DME dead time is
a period of blanking of the transponder receiver during which no incoming signal is
accepted. In most of the DME transponders the dead time is adjusted to 50 1s. If the
reflecting points are within 5 NM from the DME station then most of the echo pulses will
be rejected by the DME receiver. However, the long distance echo pulses, if they are
strong, may cause problem. The following illustration clarifies the above statements.
3.8.2.1 Short distance echo
The echo pulses may arrive in phase or out of phase compared to direct pulses. The
Fig.13 illustrates the situation when both pulses arrive in phase. If the first pair of the
echo pulse arrives with a delay of, say, 10 s then the first direct pulse will not be
distorted. However, the second direct pulse will add up with the first echo pulse. From
the above it is seen that after addition the width of the second pulse gets wider. If it is
more than 4 s then the DME receiver will reject the pair. Similarly, when the echo pulse
will arrive anti-phase then the composite waveform will be less than 3.5 s, which will
again be rejected by the receiver. To avoid this situation blanking of receiver for some
time is necessary, which is referred to as DME dead time or receiver dead time. During
this period no other pair is accepted until a reply has been made in response to that
particular pair. If the receiver dead time was not there any echo pulse that will arrive
during that period would have been accepted by the receiver. This would have created
either rejection of valid pulses due to signal deformations or false distance indications
due to echoes.
1-st pulse 2-nd pulse
Direct pulse
Echo pulse
17. 119
Resultant pulse
3.5s > 4.5s
<
12 s
(Deformation of pulse pair due to echo. Fig. 3.13)
Receiver dead time is normally adjusted to 50 - 60 s. It will protect from echo signals
that will generate from reflections closer to DME station (up to 5 NM). These are the
short distance echoes.
3.8.2.2 Long-distance echo
Long-distance echoes are those which arrive after the receiver dead time. Normally the
long distance echoes are weaker. Therefore, they are below the receiver threshold point
and rejected by the transponder. However, sometimes the far distance echoes may be
strong enough to be accepted by the receiver and trigger the replies causing false lock on
problem. To avoid the situations the receiver dead time may be increased further more
than the normal 50 to 60 s.
By increasing the receiver dead time false lock on problem may be reduced but this will
affect on overall reply efficiency of the Ground station. This is because during the dead
time the transponder receiver will reject all the incoming signals from other aircraft.
Reply efficiency is a factor that indicates the ability of the transponder receiver to receive
interrogations and make replies successfully. There is a relationship between efficiency
and dead time.
Reply efficiency = 1 - 2700X receiver dead time.
For 50 s receiver dead time we get:
Reply efficiency = 1-2700X50. 10-6 = 0.86.5 (86.5%)
For 100 s receiver dead time we get:
Reply efficiency = 1-2700X100. 10-6 = 0.73 (73.5%)
Thus, by increasing the receiver dead time while we can suppress the long distance
echoes, we reduce the reply efficiency of the ground system. So length of the receiver
dead time should be taken in to consideration only after examining the nature of the
echoes. The following illustration in Fig. 14 shows the relationship between receiver dead
time and reply efficiency of the DME. If the dead time is more than 150 s then the reply
efficiency in practice will be 50%, which is the lower threshold of an aircraft interrogator
to maintain the distance information. ICAO recommends to keep the dead time not
exceeding 60 1s unless the long distance echoes are too prominent to be neglected.
18. 120
Even then it should be increased only by the minimum amount just necessary to allow the
suppression of echoes
Reply Eff.
100%
80%
50%
60 80 100 120 140 Rx dead time s
(Receiver dead time vs. reply efficiency, Fig.3.14)
Another factor, that affects the reply efficiency, is the receiver sensitivity or receiver
threshold. In order to accept most of the aircraft signals the receiver sensitivity of the
ground equipment should be very high. In any case, if the incoming pulse pair strength is
- 120 dbW/m2 the transponder will reply with an efficiency of 70% or more. The
transponder output power is normally kept at 1KW pulse peak power.
3.9 Siting requirements
The basic requirements in siting a DME beacon are to ensure adequate coverage and to
avoid the possibility of interference to the correct operation of the aid. Site selected in
open country should keep hills, mountains, large buildings, etc. at as small angle of
elevation as practicable. The Fig. 15 shows the basic site requirements of a DME station.
Non-metallic
objects
Metallic
objects
DME 2.5 1.2
200' 1000'
Gradient of 4:100
(Basic site requirement of a DME, Fig. 3.15)
19. 121
The distant obstacle horizon should preferably not extend above an elevation angle of
0.5 when viewed from the near ground level at the proposed location of the DME.
Within 200' from the DME antenna the area should be flat and clear of all obstructions.
No group of trees or overhead lines are permitted within this radius. Beyond 200' a
downward slope of 4:100 is permitted. Within 200' - 1000' from the DME all metallic
objects should not subtend an angle greater than 1.2. For non-metallic obstructions up to
2.5 is allowed. As a general guidance, small buildings, power and telephone lines and
fences can be tolerated within 200' provided they are not higher than the DME antenna.
Normally a DME antenna is kept up to a height of 20' from the ground if that clears local
obstructions.
Large buildings such as multi-story buildings, steel bridges, metallic towers etc. are
potential sources of interference. If they are within 3 NM from the station they may cause
signal deformations. All the houses within 1000' should be constructed lengthwise and
along the radials from the DME station as far as practicable. DME is highly affected by
electrical noise. Therefore, any high-tension line above 22KV should be kept as far as
3000'. There are no restrictions on vehicular movements around the site.
3.10 Antenna system
Since DME suffers from echo signals generated by multi-path effect, highly directional
antenna system is used to avoid unwanted reflections. The DME signal is vertically
polarized. In non-directional stations, such as in VOR/DME, the radiated signal is
Omnidirectional with slightly tilted beam width of approximately 6. This provides
desired power on the horizon necessary for minimum echo generation. See fig.16.
6
DME station
(Radiation pattern of DME. Fig. 3.16)
To achieve such a low beam width stacked biconical antenna radiating elements are used.
They form together an antenna array, which provides narrow radiation pattern of 6. The
difference between maximum and minimum azimuth points is not more than 2db.
20. 122
(Biconical antenna element. Fig. 3.17)
When a DME is installed with an ILS highly directional antenna system is used.
Furthermore, in such an installation the transponder time delay is adjusted in such a
manner that the aircraft interrogator indicates zero range at a specified point.
3.11 Monitoring and calibrations
The DME is a highly accurate and dependable aid, which provides distance information
to the aircraft. Therefore, the independent monitor units constantly monitor its
performance. Normally up to two monitors are used. In the even that any of the
conditions specified below occur, the monitors will cause the following actions to take
place:
# a suitable indication shall be given at the aircraft cockpit.
# the operating transponder shall be automatically switched off and the standby
transponder will be turned on.
# The monitors continuously measure the following radiated parameters of the DME:
# a fall of 3db or more in transmitted power output.
# Pulse spacing of 12 s exceeds more than 1s
# Reply delay exceeds by 1s
# Reply efficiency 70%
# Identification tone not repeated every 30 seconds or transmitted continuously for more
than 5 seconds.
# Pulse counts 850 pulse pairs per second.
Monitoring signals are obtained from the pick up probes closely placed near the antenna
elements.
Like in other navigational aid equipment, calibration is done in regular intervals, both in
the ground and air. While ground calibration is carried out by using specific measuring
21. 123
test equipment, for the flight calibration specially equipped aircraft is deployed. The
aircraft normally checks the DME coverage area, field strength, reply efficiency and
echoes in the specified routes and places.
3.12 Wilcox DME 596B
A simplified block diagram of Wilcox model 596B DME is shown in Fig. 18.
It is one of the most widely used DME ground systems in the world. Basic system theory
of this dual equipment is as follows:
In this diagram transponder No.1 (TX-1) is selected as main and the transponder No. 2
(TX-2) as standby. Each transponder is comprised of a receiver and a transmitter. With
these selections, transponder No.1 replies (RF output) pass through the directional
coupler DC1, through the contacts of Transfer Unit 6S1, through other directional
couplers DC4 and DC3 to the DME antenna. The interrogation signals from the aircraft
22. 124
are received by the antenna and routed through the same points to the transponder
receiver. The antenna and directional couplers DC3 and DC4 are not switched while
selecting transponders.
Transponder No.2 output (standby in this case) passes through DC2, through additional
contacts of S1, through directional coupler DC5 on to dummy load. Thus, the standby
transponder is also kept in ready hot condition.
The DME has two monitors and they normally operate simultaneously. During
maintenance, one of the monitors may be used to monitor the performance of the
transponder under repair, while other works with the transponder in operation. If both
monitors are operational during normal operation of the DME, they must both report the
same fault conditions, if a fault should occur, to initiate a valid alarm.
Each monitor has two distinct functions; signal monitoring and signal generation.
Signal monitoring is done during reply; i.e. when the transponder is in the transmit mode.
The signal paths for monitoring DME parameters are from S1, through DC3 for Monitor
No.1 and DC4 for Monitor No.2 via respective coaxial jumpers. While monitoring the
signals, both monitor units simultaneously monitor the radiated parameters of the pulses,
i.e. pulse width, pulse spacing, reply delay, identification, power outputs, etc., as
specified in paragraph 2.3.6. The power output parameters are supplied by the pick up
probes (monitor antennas) in the DME antenna.
Signal Generation. In this function the receivers may be considered as "known good"
interrogators. Both monitors generate the interrogation signals as by the aircraft, which
pass through the respective test jumpers, via directional couplers DC3 and DC4 and
contact switch S1 to the working transponder. The replies received from the transponder
are routed through the same points in to the receivers.
Alarm conditions: If the radiating pulses fail to meet the specified limits, an alarm
condition would be reported by both monitors which would cause transfer to the standby
transponder by changing the relay switch S1. The transponder that was main now goes to
standby where it may be serviced while the DME station remains on the air. If the
standby transponder also proves to be faulty the system will shut down. However, in the
case of standby, the system will shut down only if the delay parameters are at fault. This
will ensure the DME service while the other transponder is on maintenance. The monitors
control operation of DME control unit (transfer switch S1).
Test conditions: The faulty transponder is connected to the dummy load via directional
couplers DC2 and DC5. In this condition the transponder may be repaired and tested
using one of the monitors. While one monitor keeps on interrogating and checking replies
with the radiating transponder, the other may be connected with faulty transponder via
Test jumpers.
Reflected /Incident Jumpers are used to monitor the direct and reflected powers of each
transponder. In the incident condition the RF outputs from both transponders are obtained
23. 125
via diode detectors, which can be displayed in oscilloscope to measure the equivalent
peak voltages. The manufacturer provides a calibrated chart for each transponder that
relates pulse voltage to pulse power output. To measure the reflected power the jumper is
changed to reflected position. VSWR is not indicated directly, but can be computed from
incident and reflected power measurements.
CHAPTER - 4
24. 126
INSTRUMENT LANDING SYSTEM
(ILS)
4 INSTRUMENT LANDING SYSTEM (ILS)
4.1. ILS as a landing aid
The Instrument Landing System, abbreviated as ILS, is a system of electronic equipment,
which assists the landing aircraft to make straight in approach by using cockpit
indications at any non-visual meteorological conditions. ILS is a standard aid, adopted by
the members of ICAO since its development in 1940's. There are hundreds of ILS's in
operation at all modern airports throughout the world. It is still considered to be the most
reliable, most utilized and most implemented precision approach system in the world.
The system comprises of a Localizer, a Glide Slope, and two to three Marker Beacons.
The landing path is determined by the intersections of two planes, as shown in Fig.1-A,
and could be explained as follows:
# A vertical plane containing the runway centerline, is defined by a VHF
25. 127
transmitter, called Localizer (LLZ).
# A horizontal plane of 2º-4º vertical angle containing the runway
centerline, is defined by an UHF transmitter, called Glide Slope (GS).
# Vertically radiated VHF Markers (IM,MM & OM) transmitters provide fixed distance
information
Marker pattern Localizer pattern
LLZ GS IM MM OM
Glide Slope pattern
(Radiation pattern of an ILS)
Fig.4.1
All these stations form a system that provides an electronic passage, exactly at an
approach angle that is required for a safe landing.
The ILS helps to bring the aircraft safely down to a pre-defined height, called the
Decision Height, from where the pilot has to make his own decision whether to land or to
make a missed approach. The missed approach is an aviation terminology for
unsuccessful landing. In this case, the aircraft has to make a turn and try to land once
again. In category- IIIC, visibility is not needed and a blind landing can be made using
electronic equipment. Therefore, based on decision height and the visibility of the
runway, three categories of ILS are defined by the International Civil Aviation
Organization (ICAO) which is tabulated below.
Table -T1
ILS CATEGORIES DECISION HEIGHT
(M)
VISIBILITY (M)
26. 128
CAT - I
CAT - II
CAT - IIIA
CAT - IIIB
CAT - IIIC
60
30
0
0
0
800
400
200
50
0
Since the pilots fully rely on ILS guidance for landing, the signals radiated by an ILS
should be very accurate and authentic. ICAO Annex-10 specifies the necessary technical
tolerances that have to be maintained for the above three categories of ILS's.
In many occasions, where the geographical conditions do not permit installation of
Marker beacons at predefined distances, Distance Measuring Equipment (DME) is co-
located with ILS. The DME can be co-located with an ILS in the following three ways:
- DME with Glide Slope
- DME with Localizer
- DME installed independently.
Thus, while landing on ILS, a pilot determines his position from the runway end by
DME. In some locations, an NDB is installed on the centerline of the runway in stead of a
Marker. Such an NDB is then called a compass locator.
Coverage of an ILS
Localizer: The horizontal localizer coverage sector is extended from the center of the
localizer antenna array to the distances of:
# 46.3 km (25 NM) with 10 from the from course line.
# 31.5 km (17 NM) between 10 and 35 from the front course line.
# 18.5 km (10 NM) outside of 35, only if the coverage is needed.
Here, the course line means the extended centerline of the runway. In most cases the
coverage is limited to 35 only. Where the topographical features do not permit a
longer range, the localizer radiation can be reduced to 18 NM instead of 25 NM within
10, and 10 NM instead of 17 NM between 10 and 35 lines.
27. 129
35
10NM
LLZ antenna 17NM 10
25NM Course line
10
35
( Horizontal coverage of localizer)
Fig.4.2
At the above-mentioned distances, the localizer signals should be receivable at the height
of 2000' and up to an angle of 7 as measured from the end of the runway. The vertical
and horizontal coverage areas of a localizer are illustrated in the figures 4.2 and 4.3
coverage
2000'
7
runway
centerline
D
(Vertical coverage of a localizer)
Fig 4.3
Here "D" is the distance from the runway end (threshold), which could be 25NM, 17NM
or 10 NM, depending upon horizontal coverage, as per fig.4.3
Glide Slope: The Glide Slope station provides coverage in sectors of 8 from the
centerline of the runway to a distance of at least 18.5 km (10NM) up to 1.75 and down
to 0.45.
Here, = Landing angle (approach angle) of the aircraft. Most of the commercial jet
aircraft land at an angle between 2 to 4.
Unlike a localizer, the Glide Slope transmitter provides horizontal coverage only up to
8. More than 8 from the runway centerline do not make any sense for landing. For a
localizer, since it gives horizontal guidance, it will direct the aircraft towards the
approach line from any angle, left or right. That's why the coverage could be wider. The
vertical and horizontal coverage of the Glide Slope are illustrated in the figures 4.4 & 4.5
28. 130
Runway 8
centerline
8
10 NM
(Horizontal coverage of a Glide Slope)
Fig. 4.4
centerline
1.75
0.45
(Vertical coverage of a Glide Slope)
Fig.4.5
Marker Beacons: Normally, only two Marker beacons are installed in most of the
locations. These are, Middle Marker MM) and Outer Marker (OM). The Inner Marker
(IM) may be added whenever its need may be felt at any particular site.
All the Markers radiate vertically in an elliptical shape on the course line through which
the aircraft makes the approach. The Marker beacon system should be adjusted to provide
the coverage over the following distances, measured on localizer and glide slope
intersection:
a) Inner Marker: 15050m (500ft 160ft)
b) Middle Marker: 300 100m (1000ft 325ft)
29. 131
c) Outer Marker: 600 200m (200ft 650ft)
In some locations, where it is not possible to install a Marker on the centerline of the
runway, it may be slightly offset, and the antenna may be tilted. In such a case the
equipment and the antenna must be adjusted so as to receive the same coverage as
mentioned above. The coverage diagrams of the Marker beacons is illustrated below in
Fig.4.6 and 4.7.
600 200m
15050m 300100m
Runway IM MM OM
(Marker coverage)
Fig. 4.6
coverage
(A tilted Marker)
Fig.4.7
4.2 Siting Requirements
Since ILS transmitters work on mid-VHF band and they carry sensitive navigation
signals; care must be taken for its proper siting requirements. The ability of an ILS
system to provide a reliable signal depends upon proper formulation of radiation pattern,
and absence of natural and man made objects that may cause signal aberrations.
In the early days, installation of an ILS was a major problem restricting its use only to the
airports with clear and plain terrain with little or no man made obstructions in the
30. 132
vicinity. Nowadays, due to advancements in technologies, companies developing and
manufacturing ILS have improved the design and performance to such an extent that an
ILS can be installed even at a very difficult location. Especially, in the last one decade
ILS technology has progressed to include important innovations in the field of antenna
and microchips, which made it adaptable to any siting challenge and more reliable than
the old systems.
Since the quality of the radiated signal is highly dependent on topographical features
surrounding the airports, the problems are successfully overcome these days by utilizing
special antenna configurations, and the manner in which RF signals are fed to them.
Therefore installation may vary significantly from site to site. As a general rule, the
following siting criterion is adopted.
Localizer siting criteria
The preferred location for a Localizer is on the centerline of the runway beyond 300
meters from the stop end. The distance between the end of the runway and antenna
positions may be varied to suit a particular condition. The antenna array could even be
located beyond 600 meters to allow for planned runway extension. However, it will
reduce the course width of the Localizer. On the other hand, it is recommended not to
reduce the distance by less than 300 meters, as antenna might be subject to high intensity
jet propulsion, and at busier airports field measurements may have to be restricted.
The shelter containing electronic equipment is generally located at 60 to 90 meters to one
side of the antenna system. The shelter could be located behind the antenna also if this
has advantage for a particular site and does not infringe airport clearance. However,
screened antenna array must be used at such installations and back-course coverage will
not be possible.
The ground surface within the site area should be as flat as possible, normally up to 1-%
gradient. Beyond the site area slope should not be more than 5%. Within the airport
boundary area at plus or minus 10 degrees from the antenna along the centerline of the
runway, there should not be any large buildings, power lines, MF or HF antennas, or
other potential reflectors.
5% gradient
300m
31. 133
180m 10 no large
objects
Centerline Ant.
60-90m 10
LLZ Building
Critical area (1% gradient)
(Critical area of LLZ site)
Fig.4.8
The antenna array is normally installed at 2 to 3 meters above the ground so as to have a
clear line of sight to a point 6 meters above the far end of the runway. The antenna
height, however, could be much more than 3 meters above the ground to suit a particular
runway. Depending upon a site condition, one of the following two Localizer systems is
selected:
Single Frequency Null Reference Localizer: Used when the terrain is flat with no
obstructions, either adjacent to or in front of the runway, that might cause deformation of
signal.
Dual Frequency Capture Effect Localizer: Used when the terrain is other than above, and
buildings or other obstructions are present in the vicinity, that might act as a reflector.
Glide Slope siting criteria
The Glide Slope antenna array is mounted in a tower, approximately 250-300 m beyond
the threshold (end) of the approach runway, and is off set by approximately 120-150m
perpendicular to the centerline. These distances, however, may change significantly
depending upon the approach angle and the site conditions. The optimum location of a
Glide Slope antenna is determined during the survey. The electronic equipment is located
in a shelter behind the Glide Slope antenna tower.
32. 134
The ground surface in front of antenna mast should be flat for approximately 700-900 m
with lateral and longitudinal gradients not exceeding 1%. The edges of the site area
should be graded to natural surface at a slope not exceeding 5%. The site should be
located on one side of the runway, remote from existing or planned taxiways, apron or
holding areas. Ideally, there should be no fixed obstructions in the site area. Isolated
buildings not exceeding an elevated angle of 0.5 degree above ground may be permitted.
Normally, standard airport clearance restrictions provide sufficient protection for
obstacles beyond the airport boundaries. However, it is preferred that up to 5 miles the
terrain should not change drastically. For example, rapidly falling or rising terrain,
irregular terrain, etc. Since, from 5 to 8 miles from the runway end is the most active
area, for landing, any signal deformation due to terrain is not desirable.
90m 250-300m Critical area (1% gradient)
5% gradient
120-150m
700m - 900m
(Critical area of Glide Slope)
Fig.4.9
The siting of a Glide Slope is more critical than a Localizer. Therefore, a careful selection
of electronic system, antenna configurations and necessary earthwork should be studied
during the site evaluation.
At many places, an ideal site is not possible to get. Therefore, depending upon the
conditions of a location, one of the following three systems can be used:
Single Frequency Null Reference Glide Slope : Used in ideal siting conditions.
Approximately, 900 m of flat reflection terrain is needed in front of antenna. Foreground
should not be sloping or rising. There should be no obstructions present in the vicinity.
Dual Frequency Capture Effect Glide Slope : Used when there is upward slope within 8
KM in the foreground of the runway. A flat terrace of 400-700 m is required in front of
antenna.
33. 135
Sideband Reference Glide Slope : Advantageous in a sloping terrain with an available
terrace as short as 300m. Antenna height is kept lower than the null reference which
allows to install it as nearer as 75m from the centerline.
Marker Beacons siting criteria
The Marker antenna is mounted on a tower, approximately at 3-5m above the ground on
the extended centerline of the runway. There is no such strict siting restriction as for
Localizer or Glide Slope. However, it is recommended not to have any metal buildings,
power lines or trees within 30m of the antenna. The markers are located as follows:
Outer Marker (OM): From 3.5 to 6 NM (7.2 - 11.1 KM) from the ILS runway threshold.
The Outer Marker marks the point where an approaching aircraft on proper heading and
correct altitude should intercept the Glide Slope and begin final descend to land.
Middle Marker (MM): It is located at 1050 150m from the threshold where the Glide
Slope angle intersects the decision height point for CAT-I ILS.
Inner Marker (IM): From 75-450m from the threshold at a point where the Glide Slope
signal intersects the decision height for CAT-II and CAT-III ILS. Inner Marker is not
used for CAT-I ILS.
4.3 General transmitting techniques
All five stations generate and radiate RF energy independently in three different
frequencies. When a pilot tunes to an ILS frequency, which is the localizer frequency, all
others are selected automatically. For a certain localizer frequency, a Glide Slope
frequency has already been determined. ICAO Annex-10 provides full account of the
assigned frequencies and the way a Localizer has to be paired to a Glide Slope. All
Markers work in the same fixed frequency of 75 MHz.
Localizer
The Localizer radiates a VHF signal capable of guiding an aircraft to the centerline of the
runway. This is accomplished by radiating a horizontally polarized signal at an assigned
frequency between 108 MHz to 112 MHz.
34. 136
The localizer antenna array radiates two different signals simultaneously. One of these
two signals is referred to as Carrier plus Sideband (CSB). It is VHF carrier wave
amplitude modulated to equal depth of 20% each by two audio tones of 90 Hz and 150
Hz. It is also modulated 8 to 10% by 1020Hz identification tone coded to the station
frequency. The identification tone does not contain any navigational information, but
simply provides station identification in Morse code.
The other signal radiated by the localizer is called Sideband only (SBO). It is a double
sideband suppressed carrier signal equally modulated by 90 Hz and 150 Hz. However,
the phase of 90 Hz signal in SBO is displaced by 90 than in CSB.
The CSB signals are fed to the different pairs of antenna array with different amplitudes
but with equal phase. This will create maximum lobe on the runway centerline providing
maximum field strength on the centerline. The signal level will decrease both sides when
moved away from the centerline, and eventually will come to a null at certain angle from
the localizer array. See Fig.1 -K. The width of the CSB signal is dependent on spacing
between antennas, number of antenna elements, and amplitude of energy distribution to
them. Normally, 7 or 14 antenna array is used. More complicated antenna system may
also be used.
The SBO signals are fed to the pairs of antennas with equal amplitude but 180 out of
phase. The amplitudes are also varied between the pairs. The antennas fed in this manner
will produce a null on the centerline of the runway due to canceling effects of signals
(because of phase difference), and will produce two lobes on both sides. The angle, at
which these two lobes are formed, depends upon the spacing of the antennas.
Since the carrier frequency of both CSB and SBO signals are the same, both signals add
up in the space creating Difference in Depth of Modulation (DDM) between 90 Hz signal
and 150 Hz signal that will vary from place to place.
The greater the relative SBO signal level is with respect to CSB signal, the greater will be
the difference in depth of modulation (DDM). Where there is no SBO signal, such as on
the centerline of the runway, there will be no difference in depth of modulation. I.e. on
the centerline, DDM = 0. The figures 4.10 through 4.12 illustrate CSB, SBO and
formation of composite DDM signals in space.
centerline
CSB
35. 137
LLZ antenna array
(CSB radiation pattern)
Fig.4.10
SBO
(SBO radiation pattern)
Fig. 4.11
DDM = 0 ( 150 Hz = 90
Hz)
150 Hz 90 Hz
90 Hz 150 Hz
(Localizer composite signal)
Fig. – 4.12
The above difference in depth of modulations is achieved due to audio signal phase
relationship between 90 Hz and 150 Hz causing canceling effect of CSB and SBO
signals. The 90 Hz is predominant on the pilot's left-hand side, whereas, 150 Hz is
predominant on the right hand side while approaching the runway. Therefore, due to
above effect, the following are achieved in the cockpit receiver:
# When the aircraft is directly above the centerline of the runway the DDM = 0.
# When the aircraft is on the left hand side of the runway, the 90 Hz modulation will
exceed the 150 Hz, and will produce a DDM proportional to angular displacement at that
point.
36. 138
# When the aircraft is on the right hand side of the runway, the 150 Hz will exceed 90 Hz
modulation, and will produce a DDM proportional to the angular displacement at that
point.
DDM = (Higher modulation % - Lower modulation %)
100
The angular displacement from the centerline of the runway remains very linear from
DDM = 0 to DDM = 0.155. In this area, for every meter left or right, the DDM is
increased or decreased by 0.00145 exactly. The angle subtended by two 0.155 DDM
points with respect to centerline is called Course Width of the ILS. By arranging the
antenna array properly, this angle is normally set to 3 - 6 left and right depending upon
the length of the runway. It is set at 107m left and right of the far end of the runway.
Beyond the course width area, although DDM is not linear, coverage is required up to
35 and is called Clearance area. The DDM relationship of a localizer is illustrated
below in Fig. 4.13
35 DDM0.155 10
DDM> 0.155
DDM=0.155
107 m
Ant.
DDM=0
Course width
(Localizer DDM sensitivity)
Fig.4.13
4.4 Vector explanation of signal formation
Fig.4.14 shows a simplified general block diagram for ILS-LLZ. The same principal
applies to ILS-GP system.
LLZ
TX
90
Hz
MOD
37. 139
(-) SBO
Hybrid Hybrid m90 = m150
=20%
CSB
(+) SBO
90Hz @ 0º
150Hz @ 180º
Attenuator & Phaser
(Fig.4.14 Simple LLZ Block Diagramme)
A simple explanation of the principals of the 3 antenna Localizer system corresponding
to Fig. 4.14 is explained as follows. Three LLZ antennas are placed at half wavelength
distance from each other and fed with (+) SBO and (-) SBO to the outer antennas and
CSB to the center antenna. Here (+) and (-) indicate that the relative RF Phase of SBO
signals are 0º and 180º apart. Also, with reference to 90Hz signal in CSB the signal in
right hand side is +90º apart.
Referring to Fig. 1-Q A through D, the principal of rotation of the SBO vectors
corresponding to the position and distance for the aircraft with respect to centerline can
be easily understood. When the aircraft is on the centerline, both the upper and lower
sidebands of the 90Hz and 150Hz SBO vectors will cancel each other. Therefore,
modulation depth of 90Hz will be equal to the modulation depth of 150Hz. Hence the
difference in depth of modulation DDM = 0. As the aircraft moves from the centerline, a
phase difference will occur and the SBO vectors will retard or advance in phase
relationship to each other.
In a Localizer:
a) 90 Hz signal in SBO is displaced by 90 than in CSB, and
b) 90Hz modulated signal is always in phase opposite (180º) to the 150Hz modulated
signal in SBO signal.
On the left hand side of the runway when looking from the site of the localizer antenna
down the runway the 150Hz modulated part of the SBO signal is in phase with the CSB
signal while on the right hand side the 90 Hz modulated part of the SBO signal is in
phase with the CSB.When the aircraft is positioned to the right of the centerline, as seen
38. 140
from the aircraft, the resultant 150Hz SBO will be in phase with the CSB signals. If the
aircraft is positioned to the left, the 90Hz SBO signal will be in phase with the CSB
signal. Thus, the vectors of 150Hz and 90Hz will add and give an increased DDM as
distance is increased from the course line. The above mentioned principals are exactly the
same for the Glide Path systems.
CL CL CL
(A)
Delay Delay
Advance
Advance
Signal received to the Signal received Signal
received to the
Right side of CL. On the centerline left side
of CL
CL CL CL
CSB 150 90 150 90 150
90
(B) vectors
Delay Advance
150 Resultant
90 Resultant
SBO 150L 150R 90R 150R 90R
90L
vectors
(C) 90R 90L 150L 90L 150L
150R
39. 141
90 Resultant
150 Resultant
150 > 90 90 = 150 90 > 150
Sum of
CSB & SBO
(D)
(Fig. 4.15 Vector representation of mixing of CSB and SBO vectors
in space)
Glide Slope Signal formation of the Glide Slope transmitter is similar to the localizer
except it radiates in UHF frequency band from 328 MHz to 336 MHz. The signal feeding
technique is also somewhat different. The Glide Slope frequencies are paired to Localizer
as follows:
LLZ (MHz) GS (MHz)
108.1 334.7
108.3 334.1
108.5 329.9
108.7 330.5…and so on.
The transmitter generates the CSB and SBO signals same as in Localizer. These signals
are also modulated by 90 Hz and 150 Hz tones. Depending upon types of Glide Slope,
two to three UHF antennas are installed one top of the other on a mast. Signals are fed to
these antennas in such a way that the composite signal in space creates DDM same as in
localizer which vary with height. On the approach slope DDM=0. Above the slope, 90
Hz is predominant, whereas below the slope 150 Hz is predominant. The above
conditions are achieved in all three types of Glide Slope system regardless of difference
in feeding of RF signals.
DDM= 0.175
90 Hz > 150 Hz DDM=0
40. 142
Decision height 0.24
0.24 DDM=0.175
150 Hz > 90 Hz
centerline of runway
Runway end
(Glide Slope DDM sensitivity)
Fig. 4.16
Here is the landing angle of the aircraft. From DDM=0 to DDM=0.175 the angular
displacement remains pretty linear. This is the most active area with respect to landing.
In different systems of Glide Slope equipment signals are fed to the antennas in the
following manner:
Null reference Glide Slope
The null reference is good for the flat terrain without any obstructions in the foreground.
Signals are fed to the two antennas as follows:
SBO - from the upper antenna. GS
CSB - from the lower antenna
Flat land
Dual Frequency Capture Effect Glide Slope
Two different transmitters generate two sets of RF signals. Frequencies of these two
transmitters are 8 KHz apart, and they are called Clearance transmitter and Course
transmitter. The clearance signal is radiated close to the airport area and is modulated
with 150 Hz only, which gives fly up signal in the lower angles. The stronger and more
concentrated signal (Course signal), is radiated at higher angle which is free from
reflections from the nearby obstructions.
These two frequencies, being only 8 KHz apart, are within the IF bandwidth of the
receiver. Therefore, the receiver will pick up only the stronger signal. This phenomenon
is called capture effect. Hence the system is called capture effect Glide Slope. The dual
frequency system has better immunity from reflections than the single frequency. See
fig.---- In dual frequency system three antennas are used. The signals are fed in the
following manner:
SBO and Clearance signal - Upper antenna
41. 143
CSB and SBO - Middle antenna GS
CSB,SBO and Clearance - Lower antenna
Rising foreground
Sideband reference Glide Slope
Two antennas are used. Heights of the antennas from ground are relatively lower than the
above two systems. The system is ideal for the sites with dropping terrain and relatively
small flat area around the antenna.
SBO signal - Upper antenna
SBO and CSB - Lower antenna
GS
Small terrace and
dropping terrain
Marker Beacons
Marker beacons are relatively simple equipment. All three types work in the same
frequency - 75 MHz. The signal is horizontally polarized. Depending upon the Marker
type, the carrier is modulated with different tones with modulation depth of 95%.
Inner Marker - 3000 Hz
Middle Marker - 1300 Hz
Outer Marker - 400 Hz
The audio frequency modulation is keyed without an interruption to the carrier to identify
the particular Marker Beacon. The keying is accomplished in the following manner:
# Inner Marker - 6 dots per second continuously
# Middle Marker - 6 dots and two dashes per second continuously.
# Outer Marker - 2 dashes per second continuously.
The signal is radiated vertically using directional antenna. Normally one to two Yagi
antenna is used depending upon coverage needed.
42. 144
4.5 Standard ILS equipment and Antenna System
LLZ Equipment
A simplified block diagram of localizer equipment, made by Wilcox Company of USA
for Cat-I and Cat-II operations, is shown in Fig 2.4-R. It is a single frequency Mark-II
model with dual transmitters. Both transmitters generate the course CSB and SBO signals
independently on the assigned frequency. It is the dual system therefore one is assigned
as the Main whereas the other as Standby. The signals are equally modulated by two
navigational tones, 90Hz and 150Hz at a modulation depth of 20% each, and at
approximately 5% by 1020 Hz station identification code. While the selected main
transmitter is on the air, the standby remains in hot condition by discharging the energy
into a dummy load.
Both transmitter outputs are connected to the Antenna Changeover Unit, which routes the
CSB and SBO signals to the Antenna RF Distribution Network. Here CSB and SBO
signals go through a series of hybrid couplers, power dividers and combiners to achieve
desired amplitudes and phases of the signals. These signals are fed to the respective pairs
of antennas to produce a composite ILS radiation pattern.
To monitor the radiated signal, there is a detector inside each antenna. Sampled signals
from each antenna are fed to the RF combining Network. This unit provides two outputs,
CSB and SBO. The CSB output provides the linear sum of all signals from each antenna.
The SBO output provides the difference of signals from the left and right antennas. After
separation of CSB and SBO signals by the RF combining Network, they are fed to the
Monitor Combining Network.
In Monitor Combining Network, the CSB signal is divided into two equal parts by a
power divider. One CSB output is used as Position RF Signal to check if there is any
43. 145
deviation in course position (0 DDM). The other portion of CSB is mixed with SBO to
produce a Width RF Signal to monitor the ILS width (0.155 DDM). The width and
position rf signals are then sent to the respective Integral Detectors where the equivalent
low voltage signal is derived for evaluation by the Monitors.
The Monitors are preset to the standard limits recommended by the manufacturer and
ICAO. Should a parameter exceed any preset limit and observed by both monitors,
alarms are initiated to the Control Unit to cease the operation of that transmitter and turn
ON the standby. Apart from signal monitoring, the Monitors also detect the cable faults,
and antenna misalignments. In such an event, a 4.5 KHz tone is generated by the Cable
Fault Detector and fed to the Monitors to command a shut down action. The station is
normally linked by a cable or by radio to the Remote Control Unit at the control room of
the airport. From here a technician can monitor the performance of the Localizer.
LLZ Antenna
The localizer antenna array radiates the rf energy generated by the localizer transmitter to
produce a VHF signal, in space, which contains modulation information that can be used
for laterally guiding an aircraft in to accurate alignment with the centerline of an airport
runway during an approach, and for landing under instrument flight conditions.
The localizer antenna array consists of 8 or 14 log-periodic dipole antenna elements. The
localizer antenna array uses 8 log-periodic antenna element for narrow aperture system
(narrow course width) and 14 elements for wider aperture system (wide course width).
Each antenna is mounted approximately 6' above the ground. This height, however, may
vary from site to site. The log-periodic antenna array, which is 9' long and 4' wide,
consists of seven horizontally polarized parallel dipole radiators that are fed from the
common balanced transmission line.
The Wilcox Mark-II systems uses the log-periodic dipole antenna array (LPDA) for the
following two specific reasons. First of all, the LPDA consists of dipoles of various
lengths, which make it independent of frequency within the specified range of 108 - 112
MHz band. Secondly, due to phase relationship between the elements the patterns from
the individual dipoles add together forming a highly directional pattern. Feeding system
in a LPDA is shown in fig. 4.17
44. 146
(Feeding system in a LPDA)
Fig. 4.17
Each Wilcox Mark-II LPDA contains a small coupler, which samples the radiated signal
approximately, 10 db down, which is routed to the monitor units in the equipment cabinet
for signal evaluation. To achieve the desired pattern, the antennas are fed with some
definite amplitude and phase difference.
GS Equipment
Generation of signal in a Glide Slope is the same as in Localizer. Only the feeding to the
antennas is different. Section 2.4.3 explains in detail the method of feeding CSB and
SBO signals to the Glide Slope antenna. As the single frequency localizer and Glide
Slope systems are the same, for diversification, here the sideband reference glide slope
system has been explained.
Sideband reference Glide Slope system is ideal for the dropping terrain and for a site with
small-leveled area. Antenna height is kept low. It is a Wilcox Mark-II GS system capable
of generating signal for Cat-I and Cat-II operations. See the block-diagram Fig.2.4-U.
The system is dual therefore there are two transmitters. Either one can be selected as the
main or standby. The transmitters generate two signals in the assigned carrier frequency -
SBO and CSB. The signals are equally modulated by two audio tones, 90Hz and 150Hz
at a modulation depth of 40% each. Both transmitter outputs are connected to the
Antenna Changeover Unit, which routes the main CSB and SBO signals to Sideband
Reference Amplitude Phase Control Unit (APCU).
From APCU the signals are fed to the GS antennas for transmission. The monitoring
probes inside the antennas sniff the signals, which pass through the respective integral
detectors. The integral detectors generate equivalent audio signals, like in localizer, for
Width and position measurement by two independent Monitors. There is one separate
monitor antenna installed at some distance to monitor the signal deviation of the path
angle.
If there is any changes in monitor parameters, the monitors will trigger the control unit to
shut down the operation and transfer to standby. Like in Localizer, the remote
control/display unit monitors the performance of the Glide Slope station.
Antenna system
45. 147
In all three systems dipole arrays are used which are installed with corner reflectors.
Depending upon the system used for GS, the antennas are mounted at different heights.
Fig.4.18 shows the typical GS antenna system.
GS Antenna system
SBO
SBO
CSB 10 7.5
CSB & SBO
5
0.25
(Null Reference GS. Fig 4.18A) (Sideband Reference GS Fig 4.18B)
SBO
CSB & SBO 15
46. 148
CSB & SBO 10
5
(Capture Effect GS. Fig. 4.18C)
4.6 Markers and antenna system.
The Marker beacons are relatively simple equipment. A block-diagram of a typical
Marker is shown in Fig.2.4-Y, which is self-explanatory. The antenna used for a Marker
is generally an Yagi- antenna. Depending upon the pattern to be used, it can be single ,
dual or tilted. The following Fig.2.4-W and 2.4-X show the radiation pattern from
different installations.
Monitor
(V-Yagi ) Fig.2.4-W (radiation
pattern)
47. 149
Monitor
(Single Yagi) Fig. 4.19 (radiation pattern)
Monitoring and calibration
Monitoring
ILS being a precision approach aid, the respective monitors continuously monitor its
performances. The following parameters are monitored by the Monitors.
Localizer : (a) Course position (centerline of the runway, DDM=0)
(b) Course width ( DDM= 0.155)
Glide Slope: (a) Course Position (Set slope angle, DDM=0)
(b) Course Width ( DDM= 0.175)
Apart from above the monitors also monitor:
(a) Reduction in RF output by 3 db.
(b) Modulation levels of 90Hz and 150Hz
(c) Identification
(d) Radiation of false signal
10 seconds for CAT-I
5 seconds for CAT-II
2 seconds for CAT-II
48. 150
Calibration
All ILS equipment have to be continuously monitored in the ground to check its
performance at a prescribed interval. At least once a year, the system has to be checked
by a specially equipped aircraft to monitor landing angle, course width, centerline
alignment, and other performances.
CHAPTER - 1
NON DIRECTIONAL BEACON
(NDB)
1. NON DIRECTIONAL BEACON (NDB)
49. 151
1.1 NDB AS A NAVIGATIONAL AID
Non-Directional Beacon is a radio navigational aid used by the aircraft all over the world
for finding directions while flying from one point to other. Discovery of radio and ability
of detecting its source of emission, utilizing directional antenna, led to the development
of NDB. It is the simplest and oldest system, which has for many years played a vital role
in the navigation system for both aeronautical and maritime uses and will probably do so
for many years to come.
Non-Directional Beacon is a ground station that transmits a low frequency or medium
frequency signal, which is radiated Omni-directionally in the horizontal plane (azimuth),
with vertical polarization. There is no coded navigation information inside the signal
apart from the station identification in Morse code that repeats 7 times per minute. The
NDB receiver in the aircraft gives the pilot information of the “bearing” to the NDB
transmitter stations, which are located in the air-routes or at the airports. Bearing is the
horizontal angular displacement in clockwise direction with respect to North. In addition
to the directional information the NDB station also gives indication when the aircraft is
passing overhead a station, i.e., the NDB station provides a position fix overhead
indicated by a decrease in field strength and an abrupt change of indicator needle by
180. The NDB is widely used because they are:
# Inexpensive
# Simple electronics and easy for maintenance
# Omni-directional information
# Responsibility of accuracy mainly depends upon airborne receiver.
1.2 PRINCIPAL OF OPERATION
NDB is simply a radio transmitter that transmits tone modulated RF signal in the
LW/MW frequency band with station identification seven times per minute. Volume-1 of
ICAO Annex-10 to the convention on International Civil Aviation Organization states
that, "The radio frequencies assigned to NDB's shall be selected from those available in
the portion of the spectrum between 190 KHz and 1750 KHz. The frequencies being used
for NDB can vary from zone to zone. As the frequency band from 525 to 1605 KHz is
widely used for Radio Broadcasting, most of the frequencies for NDB's are selected
below 525 kHz within 200 to 415 kHz. The signal is amplitude modulated at 95% by a
station identification audio tone in Morse code (A2), which repeats 7 times per minute to
identify a station. The identification tone consists of two to three letters. The frequency of
the modulating tone can be either 400Hz or 1020Hz. Each letter is separated by a dash.
For example: The Kathmandu NDB at the Tribhuvan International Airport is coded as
KAM, which in Morse code translates as:
50. 152
dash dot dash dash dot dash dash dash dash
K A M
In the aircraft, a receiver called Automatic Direction Finder (ADF) automatically displays
the station bearing as soon as it is tuned to a NDB station. The Automatic Direction
Finder uses the Loop Aerial that has a specific direction finding property. Depending
upon the orientation of the loop aerial, signals in its output varies greatly. A loop aerial
possesses the following properties.
1.3 LOOP AERIAL
Direction finding maybe carried out in any region of radio spectrum, though certain
frequencies are specifically allotted for radio navigation purpose. In aviation only LF/MF
and VHF are used for radio direction finding. LF/MF are used for NDB ground stations
whereas VHF is used for finding the direction of the aircraft from the ground. The
technical features of direction finders operating in various frequencies naturally differ,
but the fundamental principles remain the same. In the LF/MF, due to comparatively very
large wave length, so called LOOP ANTENNA is extensively used. Loop Antennas are
highly directional in property, which could be derived mathematically as follows:
Consider a rectangular loop antenna of length “a” and width “b” with its plane vertical
mounted so that it can be rotated about its vertical axis. Let there be a vertically polarized
electromagnetic wave “E” incident on it, coming from a direction making an angle “”
with the plane of the loop at its center.
N
B C
b
a e2 e1 CD
b/2 ½ b Cos
A D AB b/2
Output
The source is assumed to be so far away that the incident wave is a plane wave. Voltages
are induced in the vertical members of the loop, but not in the horizontal members as the
wave is vertically polarized. The magnitude of the voltage induced in the two vertical
members is therefore a.e1 and a.e2, where e1 and e2 are the magnitude of electrical field
51. 153
in rms. The voltages in the two members will not be in phase, as can be seen from the
diagram since the arrival times will not be the same. Taking the electrical field at the
center of the loop as the reference, the voltage induced in AB lags by an angle , and that
induced in CD leads by , where being the phase difference of the arriving signal with
respect to center of the loop. Considering = 2 and difference in path length is ½ b
Cos. Then phase difference equivalent to path length is
= 2. b Cos = .bCos
2
If the electric field at the center e(t) = E Cos t then voltages induced in two vertical
members will be :
e1 = aE Cos (t - b Cos )
e2 = aE Cos (t + b Cos )
Therefore resultant voltage at the output of the loop antenna will be
e = e1 – e2 = aE Cos (t - b Cos ) - aE Cos (t + b Cos )
Or
e = 2 aE. Sin t . Sin b Cos
Since “b” is very small in comparison to then we could do approximation as
Sin b Cos = b Cos
Hence e = 2E . ab Sin t. Cos
From the above formula we could make the following conclusions:
a) Output of the loop antenna is dependent of the incident angle “”. When the plane of
the loop antenna is perpendicular to the incident radio signal , i.e. when “” is 90
the output from the loop is zero and maximum when “” is 0
b) Output from the loop antenna will increase when the dimensions “a” and “b” will
increase. That is, output is directly proportional to the area of the loop. Accordingly,
if there are “N” turns in the loop then output voltage will also increase by “N” times.
Accordingly, a Loop Aerial may have two distinct positions as follows:
Null Position
52. 154
If the plane of the loop is at right angle to the direction of the waves coming from the
radio beacon, the two sides of the loop will be at the same distance from the station. Thus
the signals will arrive at the same time without any phase difference, causing current
induced in both sides of the loop to be the same. However, since they are opposite in
direction, they will cancel each other producing no rf output from the antenna. This is the
null position of the loop aerial.
rf waves
Min. or no signal
Maximum Position
If the plane of the loop aerial becomes parallel to the direction of the waves, signals will
reach at both sides with maximum difference in phase. That will produce maximum
signal strength.
Max. phase difference
rf waves
Max. signal
The Null position is preferred in direction finding because:
# It is easy to determine a null than a maximum
# It is more accurate and sharper.
Sensing
There are always two null positions and two maximum positions for a loop antenna. The
loop aerial will always receive the same signal by turning it to 180 degrees. This may
create confusion about a station and there will be an ambiguity of 180 degrees regarding
the direction of the station.
53. 155
The ambiguity is solved in the modern aircraft receivers by addition of another non-
directional antenna for sensing. The ADF receiver uses a rotating loop antenna, which
gives the figure of eight pattern, and a fixed sense antenna that gives an Omni-directional
pattern.
The figure of eight pattern from the loop antenna has positive (+) and negative (-) phase
as indicated below. The sensing antenna has omni-directional circular pattern with (+)
phase. The composite pattern therefore will be a cardioid as shown below.
Circular pattern
Cardioid
Figure of eight pattern
When pilot tunes to an NDB station the ADF loop antenna automatically turns the
indicator towards the direction of the station with reference to magnetic north. This is
interpreted in the needle as the Radio Magnetic Bearing Indication.
1. 4 ADF DISPLAY
The Automatic Direction Finders (ADF) are manufactured with either analog or digital
display. In either case, in ADF receiver, bearing information is presented on either a
Relative Bearing Indicator (RBI) or the more complex Radio Magnetic Indicator (RMI).
+
- +
54. 156
1.4.1 Relative Bearing Indicator :
This is the simplest type of display, shows the pilot the bearing of the tuned NDB
transmitter relative to the axis of the aircraft. The RBI is measured clockwise in degrees
(O - 360) from the nose of the aircraft. See Figure above.
1.4.2 Radio Magnetic Indicator:
This instrument displays the magnetic bearing of the NDB as well as the heading of the
aircraft. Therefore it is more convenient for the pilots. The figure above shows the
method of measuring RMI.
1.5 USE OF NDB
55. 157
By using relative or magnetic bearings, NDB can be utilized for various navigation
purposes. Depending upon their use and where they are placed.
5.1 Homing: NDB is installed at the vicinity of the airport. Aircraft find their way to the
airport by tracking on to the beacon.
5.2 En-route: NDB is installed in between the airports on the prescribed routes.
Sometimes the beacon may be offset from the route. However, by using relative bearing a
position fix can be determined.
5.3 Holding: Such an NDB is called Locator Beacon and is placed a few miles away from
the airport area. Aircraft circle the beacon at different heights waiting for permission to
land.
5.4 Instrument approach: NDB is installed on the center line of the runway. Aircraft
make straight-in approach by using the NDB.
1.6 ADVANTAGES OF NDB
Although there are now several more accurate navigational systems available on other
radio frequency bands, the NDB is still used in every country in the world, and will
continue to do so for many more years to come. The reasons are obvious which can be
outlined as follows:
# Very simple air-borne and ground equipment
# Inexpensive to install and maintain
# Omni-directional information
# Any number of aircraft can use the same radio beacon
# Responsibility of accuracy mainly depends on airborne receiver
# Multi-purpose uses
1.7 LIMITATIONS OF NDB
Like any other equipment, NDB also have its own limitations. If an NDB is used under
certain condition pilots may get sometime large and potentially dangerous bearing errors.
Therefore, NDB cannot be considered as a precision aid and should be used with caution.
The principal factors liable to affect the NDB performance are as follows:
1.7.1.Quadrantal Error: Due metallic portions of the aircraft the radio waves get
deflected. Error produced by such a phenomenon is called quadrantal error because it is
maximum in all four quadrants. Quadrantal error differs for one aircraft to other, which
can be corrected by using the correction curve for that particular aircraft.
Max error Max error
56. 158
Max. error Max. error
A typical quadrantal error curve:
+10
+5
0 90 180 270 360
0
-5
-10
1.7.2. Coastal refraction:
In coastal areas the differing radio energy absorption properties of land and water result
in refraction of NDB transmissions. This causes error, known as coastal refraction. It is
most marked when transmission cross the coastline at an angle other than right angle and
when the transmitting station is located away from the coast. If the angle is less than 30
the error gets worst. Therefore NDB's in the coastal areas should be used with utmost
caution.
True bearing
NDB
57. 159
Apparent bearing
Land Sea
1.7.3. Night Effect:
At night, in addition to the interference that can occur due to transmissions from different
stations, it is possible to receive the ground wave signals contaminated by the sky wave
signals from the same station. This will give rise to bearing errors of varying magnitudes
depending on the heights of the ionized layers and the polarization of the signals on
arrival at the receiver. Night effect is especially most marked during the twilight hours
when skywave contamination can cause fading of signal strength, which will cause
wandering of the ADF bearing needle.
1.7.4. Mountain effect:
ADF receivers may be subject to errors caused by the reflection and refraction of the
transmitted radio waves in mountainous areas. High ground between the aircraft and the
beacon may increase the errors especially at low altitudes.
1.7.5. Static interference: All kinds of precipitation, including falling snow and
thunderstorm can cause static interference of varying intensity to the ADF receivers.
Precipitation reduces the effective range and accuracy of bearing information.
Thunderstorm can produce errors of considerable magnitude including even entirely false
indication. Indeed it is often said that in an area affected by thunderstorm activity, the
ADF bearing pointer would rather indicate the direction of thunder than the NDB station.
1.7.6. Lack of failure warning system:
Because of lack of failure warning devices on ADF receivers, failure of an NDB station
may produce wrong indication which will go unnoticed. Constant monitoring and
hearing of identification signal is the only way to detect the failure of the ground station.
1.8. SITING REQUIREMENTS
An NDB may be located on or adjacent to the airport. If it is used as an approach aid then
it should be located on the centerline of the runway. In any case, the siting criterion is
not very complicated. However, the following should be observed:
The NDB site should be smooth, level and well drained. The antenna system should not
penetrate the approach or transitional surfaces of the airport. There should be no metal
buildings, power lines or heavy metal fences around the NDB station at a distance closer
than 100 feet.
1.9. ANTENNA SYSTEM
58. 160
NDB antennas are similar to normal LW/MW antennas. Because of dominating
transmission by the ground wave, vertically polarization is necessary. Hence vertical
wires or self-supporting structures are the solution. Since, NDB operating frequency is in
order of only a few hundred KHz, the practical length of an antenna must be much lower
than /4 wave length. For example, for an NDB station working on 250 KHz, its
wavelength will be:
= 300/ 0.250 = 1200 meters
or /4 = 300 meters
To erect an antenna 300 meters tall is not only very expensive but also prohibited near the
airport areas due to possible obstruction to the aircraft. In practice much shorter antennas
(from 20 to 40 meters) are used. Because the antennas are relatively very short they are
always capacitive in nature. Therefore, to resonate a NDB antenna some tuning
inductance must be used. As described above, NDB antennas are vertically polarized.
Therefore the radiator is kept in vertical position from ground. The earth acts as an image
to the radiator. To increase the capacitance of the antenna, a ground radial system has to
be provided. A ground radial system, which is also called counterpoise, is a system of
copper wires buried approximately 15 cm below the surface of the ground. The size and
shape of the counterpoise will vary with the type of antenna system used. Normally the
wires are laid at 5 to 10from the center, just below the radiator. Fig. 2.1-M below
shows a typical ground counterpoise of an NDB.
1.9.1Radiation pattern
The polar diagram of an NDB antenna radiation is shown below. It is Omnidirectional in
the horizontal plane (H-plane) and directive in vertical plane (E-plane). Theoretically
there is maximum gain along the earth surface, but in practice we will have maximum
field strength at some angle from the surface due to losses in the ground wave
component.
Theoretical
Practical
(H-plane) (E-plane)
Polar diagram of NDB antenna
1.9.2 Types of antennas
A very simple, effective and widely used NDB antenna is T-antenna, which is illustrated
below. The vertical wire is, of course, the actual radiating element and the horizontal wire
provides additional antenna capacitance to the ground. To increase the capacitance of the
antenna three or more parallel wires are used in the horizontal portion. The normal height
of T-antenna is approximately 20 to 30 meters. Sometimes an inverted L-antenna is also
used. However, it is more sensitive to unwanted horizontally polarized electric field
component compared to a T-antenna.
59. 161
The self-supporting mast or a mast radiator is also a popular NDB antenna. The normal
height of such an antenna is 20 to 40 meters. Top-loaded insulated guy wires increase
capacitance. Such an antenna is more efficient than a T-antenna and therefore widely
used for long range NDB as well as MW/LW broadcasting.
For locator beacons or for the beacons used for approach purposes, since the coverage
required is very small, relatively short antennas are used. One of such antennas is
Umbrella type. It is a small self supporting mast radiator with several top-loading
elements like an umbrella. The top loading increases the capacitance of the antenna,
hence it becomes easier to resonate. The normal height of an umbrella antenna is not
more than 12 meters.
ILLUSTRATIONS
Top loading
Radiator
NDB
shelter
20 m
12.5 m 30 m 12.5 m
GND
(A TYPICAL T-ANTENNA)
(Fig. 2.1-L)
60. 162
Copper wire
buried 15 cm under ground
5 to 10
degrees
(EARTH RADIALS OF NDB ANTENNA)
Mast radiator
Top capacitance
10m
Top loading
25m
wooden pole antenna
insulator
12m
GND GND
( A TYPICAL MAST RADIATOR)
(UMBRELLA ANTENNA)
61. 163
1.10 FACTORS AFFECTING NDB ANTENNA
The radiation resistance of an NDB antenna is very low and equals to only a few ohms. If
it were possible to match a source of radio frequency energy so that all the power is
dissipated into the radiation resistance then these antennas would have been equally
efficient as one with much higher radiation resistance. However, in practice the total loss
resistance of the antenna is much higher than the radiation resistance. Therefore, most of
the energy gets wasted and the efficiency of the antenna becomes too low.
There are several factors that affect the efficiency of an NDB antenna. These are briefly
described below:
1.10.1 Antenna Reactance :
NDB antennas are capacitive in nature. The capacitance of antenna is important to know
because it provides the basis for knowing the amount of tuning inductance required for
resonance.
Since, for resonance : L =1/ C
Smaller capacitance will need bigger inductance, causing more loss of energy in the
conductor. Thus the capacitance of an antenna should be as higher as possible. This can
be done either by increase in height of the antenna, or simply by additional top-loading.
The second option is more economical and favourable. The capacitance of an electrically
short vertical antenna may be calculated by the use of well known transmission line
formula. For a simple vertical radiator (insulated from ground) having a height “H” from
the ground and diameter “D”, its capacitance can be roughly calculated from the
following formula:
C = 5766 X Tan θ
Log 2H
D
Here C in pF, H and D in feet, and θ- electrical length of the radiator in degree.
The following table gives approximate values of a vertical radiator without top loading in
pF. From this it is evident that antenna capacitance is dependent of vertical height and
diameter of the radiator element.
Antenna Height (ft)
Antenna diameter in inches
0.1 1 12
50
75
100
150
200
250
90
131
167
245
322
399
120
170
219
314
428
504
184
254
308
451
654
795
62. 164
Where antenna length is desirable to keep short, top loading is used. This greatly
increases the capacitance of the antenna thereby reducing the requirement of large
antenna tuning inductance. Additional capacitance generated by top loading in a T-
antenna can be calculated as follows:
C = 5766 X Tan (0.07315L)
Log 4H
D
Here C in pF, H, L and D in feet. L – Length of top loading wire.
1.10.2 Radiation resistance :
The base radiation resistance is another important characteristic. It is a characteristic,
which has a direct relationship to the radiated power and consequently to effective range
of the NDB. Because the NDB antennas are electrically very short (less than 30), the
current distribution along the antenna is linear and radiation resistance may be calculated
to reasonably close approximation by the formula:
R = 2 /328 , where is the electric length of the antenna in degree. = 360
With the above formula it is evident that by increasing the length of the antenna its
radiation resistance increases, and hence the efficiency increases. See following table.
Antenna Height (ft)
Radiation resistance in Ohms
200KHz 300KHz 450KHz
50
75
100
150
200
250
0.041
0.092
0.163
0.367
0.657
1.020
0.092
0.207
0.367
0.825
1.467
2.295
0.206
0.466
0.825
1.857
3.300
5.164
1.10.3 Antenna Q :
Antenna system Q is the ratio of the reactance of the antenna capacitance to the antenna
total system resistance. It is always preferable to keep the Q as low as possible to reduce
losses in the antenna system.
Since Q = Xc/R ,
Q can be reduced by increasing capacitance of the aerial. I.e. by addition of top loading
or by increasing the height. NDB transmitters are usually required to have a bandwidth of
at least 2X1020 Hz. 1020 Hz being the max ident frequency.
63. 165
Bandwidth = f/Q
Therefore, at 300 KHz Q = 300,000/2040 = 147
Which means a Q of 147 at 300 KHz NDB station will insure that the ident modulation
will be radiated without any distortion. If bandwidth of the antenna is low ( Q is high)
then instead of 1020 Hz ident modulation of 400Hz should be used.
1.10.4 Expected range
NDB antenna should be designed in such a way that it should radiate reliable signal up to
the required coverage area. ICAO has specified that in the coverage areas the field
strength should not be less than 70V per meter. Between the latitudes 30N and 30S
field strength of 120V may be required.
1.10.5 Voltage at the antenna base
The peak voltage at the antenna base for usual NDB is :
V = IA x XC Where IA - Antenna current and XC - Antenna reactance.
Example: For an NDB with T-antenna top loaded with three wires and 50 ft high and
100 watts transmitter C = 581 pF and antenna Current IA = say 7 Amps
at 200KHz XC = 1370 Ohm
then V = 9590 Volts
For the same antenna without additional top loading, C = 90 pF XC = 8842
Hence V = 61,894V
This amount of voltage is difficult to contain and would probably cause considerable
difficulty due to corona, flashover, etc. Therefore the antenna capacitance of an NDB
antenna should be kept around 500 pF or more.
Conclusion : To increase the efficiency and to improve the performance of an NDB
antenna its capacitance should be as high as possible and should be more than 500pF
at the lower frequencies. This can be achieved either by increasing the height of the
antenna or by providing additional top loading.
64. 166
1.11 TRANSMITTING EQUIPMENT
The NDB transmitter is relatively very simple equipment. The RF carrier is amplitude
modulated either by 400Hz or by 1020 Hz tone, which is coded with two to three letters
station identification in Morse Code. A simplified block diagram is shown in Fig. 2.1-Q:
Antenna
A monitor equipment monitors the performance of the radiating signal. Radiation is done
in A0/A2 mode. Depending upon the use an NDB could be classifies as one of the
following:
High Power: usable range extends up to 400 NM. Radio beacons of this type are
considered as en-route or homing radio navigational aids. The transmitter output is
normally 100W to 5KW.
Low Power : usable range extends from 10 NM to 25 NM. Radio beacons of this type are
called locators and are normally used for approach or holding purposes. The transmitter
output power is kept below 100W.
1.12 MONITORING AND CALIBRATION
Normally the NDB beacon has two transmitters and two monitors, i.e. dual equipment
system. Monitor analyzes the radiated signal and checks the following:
# Gives alarm if the transmitted carrier power is reduced more than 3dB. i.e. 50%
# Gives alarm if the identification signal is removed or continuous by any reason.
# Gives alarm if the monitor itself becomes faulty.
When one of the above conditions occurs the monitor unit commands the changeover unit
to shut sown the faulty transmitter and to start the standby. The NDB stations are
IDENT
UNIT
TRANSMITT
ER UNIT
MONITOR
65. 167
normally unattended, which are monitored for a failure by the technicians through radio.
To distinguish main transmitter from standby normally the main is modulated with 1020
Hz and the standby with 400Hz.
CHAPTER - 5
SURVEILLANCE RADAR
(Radar)
5. RADAR THEORY
5.1. Introduction
The word RADAR was originally derived from the descriptive phrase "Radio Detection
And Ranging". Although this phrase has for a long time been used, it seems to be an
incomplete description of what Radar can be used for. The present-day RADAR can
provide much more information than finding the range of an object. The fundamental
principal of all Radar systems is to calculate the distance of an object from the Radar site
by measuring the time a pulse of radio energy takes to travel to the object and back again.
The importance of radar in aviation is that it can provide information about the precise
position and velocity of the aircraft. In addition, the more complex equipment can supply
other useful data, such as, velocity, identification, height, etc. Radar can contribute to the
66. 168
safety and surveillance of the aircraft in thick density areas. For example, near an
aerodrome, where the air traffic density is very high, radar may be used to sequence the
aircraft onto final approach as a final approach aid, and for separation soon after take off.
There are two basic types of Radar system:
Primary Radar - A system, which uses reflected radio, signals.
Secondary Radar- A system in which radio signals transmitted from the
Radar station on the ground initiates the transmission
of radio signals from another station, e.g. aircraft.
A basic primary radar system is illustrated in fig.1-1 below. Pulses of radar energy are
transmitted in the desired direction. Some of the pulses of energy may encounter the
target. A portion of this energy is reflected by the target and returns back to radar
receiver. Information about this target is then extracted and displayed in a suitable display
system such as radarscope.
Antenna
Target
Display (Radarscope)
(Fig.5.1: Basic block diagram of Radar)
The basic principal of secondary radar is much the same but there is one important
difference which should be clearly understood. While primary radar employs reflected
pulses, the secondary radar requires the object to transmit its own energy. The secondary
radar systems have become more complicated than the primary radar and now they are
capable of providing much more information than the primary radar. Both primary and
secondary radar shall be dealt with in detail in the following paragraphs. Some of the
terminologies being used in Radar are:
Radar Energy: part of radio energy spectrum between about 1mm and 100cm which is
transmitted in a series of pulses of fairly short duration in the region of 5 µs.
Radar Echo: visual indication on a display of a signal reflected from an object in the
Primary Radar.
Tx. Rx.
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Radar Response: visual indication on a display of a Radar signal transmitted from an
object in reply to an interrogation in the Secondary Radar.
Radar Blip: a collective term meaning either Echo or Response.
Use of Radar in Civil Aviation
Surveillance is one of the most important elements in aviation. Through surveillance an
air traffic control post can monitor the movements of aircraft as well as can provide
guidance and avert accidents. In aviation surveillance is done in two ways:
a) Through position reporting
b) Through Radar.
Providing surveillance through position reporting by aircraft is highly unreliable and
could cause misunderstandings resulting in fatal accidents. Therefore, radar has been
widely used in civil aviation as one of the major surveillance tools for many years now.
The prime purpose is to detect the aircraft flying within the controlled as well as
uncontrolled airspace for traffic separation and control, and also for providing guidance
during landing. Some of the uses of radar in aviation are as follows:
ASR (Airport Surveillance Radar)
It is a medium to low power primary radar installed for surveillance and traffic
separation in the airport terminal area. It works in S-band with pulse power up to 1
MW and antenna revolution around 15rpm.
PAR (Precision Approach Radar)
With this radar an air traffic controller guides the approaching aircraft to take the
correct approach to the airport. PAR works in the band 9000 - 9180 MHz and
normally of low power.
ARSR (Air Route Surveillance Radar)
As the name denotes, ARSR works as an air-route surveillance radar with relatively
high coverage range of 200 to 300NM. It works in L-band (1250 - 1350 MHz) and
normally placed in the air routes with high traffic density.
5.2 PRIMARY RADAR
5.2.1 General Block Diagramme
The purpose of primary radar system in aviation is to present a continuous supply of
useful information to the air traffic controllers on the ground regarding the range, bearing
and, in some cases, elevation of the aircraft within the operational range of the radar
system. Thus, every primary radar system must be capable of:
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Transmission
Reception
Display
A basic block diagram of the primary radar is shown in Fig.1-2. Trigger Unit, which is
also called as the Master Timer, provides triggering signals in the form of a series of very
brief electrical pulses at a regular interval. Each pulse fires the modulator to send a high
power high voltage pulses to the transmitter. The duration of square wave pulses from the
modulator is determined by certain design characteristics in the modulator. The beginning
of each pulse from the modulator unit switches on the transmitter and the end of the same
pulse switches it off. Thus the modulator pulses represent a kind of on/off switch for the
transmitter.
Antenna
Antenna
movement control
Tx. RF Energy Pulses
Switching pulses
Signal
from Receiver
Trigger pulses
Sync signal
from Trigger unit
Reference Data
T/R
SW
Transmitter
Modulator
Trigger Unit
High Gain
Low
Noise
Receiver
Radar
Displa
y
Unit
Time
Base
Unit
